The present invention is concerned with a device for susceptibility testing of microorganisms comprising a sealable container, a square carrier coated with a growth medium for microorganisms and a rectangular, transparent, non-porous, inert test strip, on one side of which is applied two antimicrobial substances separated by a zone free of substance, and on the other side of which is a scale for direct reading of the susceptibility.
Susceptibility testing of microorganisms is carried out by exposing the antibiotic(s) and/or chemotherapeutic(s) to be tested to a growing culture of the microorganisms. After incubation, the size of the inhibition zone demarcating a circular area of no growth from the general lawn of microbial growth is read. The susceptibility of microorganisms for various antibacterial substances can be established in this way. This can be used as a guidance for the selection of an appropriate antimicrobial therapy for the treatment of infections caused by the microorganism in question. Susceptibility testing of microorganisms can be carried out by applying either a suspension of colonies from pure isolates or biological fluids containing the infected microorganism directly on a growth medium.
Devices for the determination of the susceptibility of microorganisms are described in U.S. Pat. No. 3,416,998, DK-C-137089 and SE-C-403382. U.S. Pat. No. 3,416,998 relates to a dried, transparent sheet of agar, in which an active substance, such as a chemotherapeutic agent or the like has been incorporated. These agar reagent sheets can i.a. be produced by the dropping of agar incorporated with the reagent on a moving sheet of plastic material. After the agar reagent sheet has dried the plastic material is removed, whereafter pieces of suitable shapes and sizes are cut and are ready to be used. These are intended to replace i.a. antibiotic discs. DK-C-137089 concerns an indicator for the determination of the susceptibility of microorganisms comprising an absorbing carrier, on which growth medium mixed with bactericides has been applied, and on one side of which a moisture-proof layer has been applied. In SE-C-403382 is disclosed means in which the active substance is applied on the growth medium with the aid of a porous carrier. The concentration of the active substance is said to vary continuously along the whole length of the carrier. From this patent is evident that the active substance is essentially transferred from the porous carrier to the cultivation medium in the longitudinal direction of the carrier, the consequence of which is that in the cultivation medium it is not possible to achieve a variation of the concentration of active substance with such an accuracy that it can be read against a pre-determined scale. According to this patent it is also possible to apply two active substances on the growth medium, but only in such a way that concentrations of the substances varying in two directions perpendicular to each other are achieved.
Susceptibility testing of microorganisms using biological fluids can further be exemplified in more detail by describing a method for the determination of the suceptibility of urinary pathogens to antibacterial substances. In this method a so called dip-slide is used, i.e. a rectangular carrier coated with a growth medium for microorganisms, e.g. a suitable agar medium. This dip-slide is e.g. attached to a lid, which fits to a tube. The dip-slide is dipped into a urine specimen, whereby the specimen with the bacteria in it is exposed to the agar surface. Thereafter one or more so called antibiotic discs are applied on the agar surface. These are small discs of a porous material, which have been impregnated with a predetermined amount of an antibacterial agent. When these discs are in contact with the agar surface the antimicrobial agent diffuses out into the agar medium. After applying the discs to the dip-slide, it is placed into a tube, which is then sealed. Thereafter the dip-slide is incubated over night in an incubator (a temperature of about 37.degree. C.) after which the dip-slide is removed from the tube. The results are read by looking for the appearance of an inhibition zone, i.e. a zone with no or very little bacterial growth, and measuring the diameter of this zone. This gives an indication on whether the antibiotic being tested may be effective for the treatment of an infection caused by the microorganism in question. This method is described inter alia in Chemotherapy 25:227-232 (1979) and The Practioner, vol. 224, 931-934, September 1980. The disadvantages with this method are among others the difficulty of handling such a technique in out-patient care centres, as the antibiotic discs due to their small size must be applied by using tweezers or a needle. Further there is a certain risk of infection when reading results of the susceptibility testing since it is necessary to remove the dip-slide from the outer tube. The size of the inhibition zone has to be measured manually and compared to a set of different interpretive templates for different antibiotics, which is a time consuming procedure. At the same time there are technical disadvantages associated with the porous antibiotic discs as diffusion of antibiotics therefrom to the agar layer does not always take place uniformly due to variations in the porosity of the paper of the agar layer and differences in the physico-chemical properties of different antibacterial agents. This is valid for all porous carriers and thus for all the above-mentioned means.